Method of making a dispersion strengthened metal

ABSTRACT

A method of making a dispersion strengthened metal or alloy in accordance with this invention consists in spraying a molten metal material and a reactive constituent on to a target. The reactive constituent is passed through an atmosphere which converts it into a material which forms a dispersion phase within the host material when the latter is deposited on the target. A dispersion strengthened metal or alloy made in accordance with this method possesses improved mechanical properties and increased strength at high temperatures.

United States Patent Darling [451 Oct. 10, 1972 [54] METHOD OF MAKING A DISPERSION [56] References Cited STRENGTHENED lVIETAL UNITED STATES PATENTS Inventor: Alf!!! y y a ng. t woo 2,129,702 9/1938 Merle ..164/46 Mlddlesex, gland 2,460,991 2/ 1949 Le Brasse et al. .29/192 R UX 73 Assignee; Johnson Matthey & Company 3,484,305 12/1969 Rose et a1. ..29/420.5 X Limied, London, England 3,138,851 6/1964 Radtke et a1. ..29/182 [22] Filed: July 1969 Primary Examiner-John F. Campbell [211 App]. No.: 861,522 Assistant Examiner-Donald C. Reiley, Ill

Attorney-Goodman and Bauer Related U.S. Application Data [63] Continuation of Ser. No. 839,985, July 8, 1969, ABSTRACT abandoned A method of making a dispersion strengthened metal or alloy in accordance with this invention consists in [30] Forelgn ApPhpatmn Pnomy Data spraying a molten metal material and a reactive con- Julyl2, 1968 Great Britain ..33,392/68 stituent on to a target. The reactive constituent is passed through an atmosphere which converts it into a [52] U.S. Cl ..29/527.7, 29/192 R, 29/4205, material which forms a dispersion phase within the 164/46, 164/76 host material when the latter is deposited on the tar- [5 Cl. ..B23k get A dispersion strengthened metal or made in [58] Field of Search..29/420, 420.5, D16. 39, 192 R, accordance with this method possesses improved V/CK E R5 PYRAMID mechanical properties and increased strength at high temperatures.

16 Claims, 2 Drawing Figures WORK HARDEN/NG HARDNESS CHARACTERISTICS OF 240 PLATINUM AND PLATINUM ALLOYS 220 IOZRhDD/UM-PLAT/NUM 0-06 WTZ HE POWDER ME TALLURG/CALLY PROCESSED PLA T/NUM r X In FLAME SPRAYED PLATINUM 0! WT %Z/RCUN/W PUREPLAT/NUM 100 /7 a0 60 7-5 20 24 TRUE STRAIN PATENTED E 3,696,502

SHEET 1 [1F 2 V/CKERS PYRAMID WORK HARDEN/NG HARDNESS CHARACTER/SW05 OF 240 PLATINUM AND PLATINUM ALLOYS 220 IOZRILDDIUM-PLATINUM POWDER METALLURG/CALLY 180 PROCESSED PLA T/NUM 0-08 WTZ 720 )i 0 9/ /U .49 y i FLAME SPRAYED 720 PLATINUM 01 WT x f Z/RCON/LM PURE PLATINUM 100 7-6 20 2' TRUE STRAlN properties and particularly increased strength at high temperatures.

With this aim in view, various methods have been proposed for making dispersion strengthened materials. One basic problem confronting metallurgists making dispersion strengthened metals is that of ensuring that the dispersed phase, for example oxides such as thoria, zirconia, hafnia, titania, alumina or the lanthanides, forms a stable array of particles in the submicron range and does not react with the host metallic environment supporting the dispersed phase.

Generally, dispersion strengthened metals and alloys are made by mixing metal or alloy powders with fine refractory particles and thereafter, consolidating the particulate mixture by powder metallurgical techniques.

Alternatively, dispersion hardened metals and alloys may be made by producing a metal or alloy melt which is atomized either in air, gas or steam jets, or by mechanical methods such as spinning tables. Depending upon the conditions involved this atomization process produces a more or less heavily oxidized powder. Another process produces oxidized aluminum powder which is still further oxidized both on the surface and internally by ball milling under oxidizing conditions. Also atomized lead used in a further dispersion strengthened process is ball milled to increase still more its oxygen content. In particular berylliumcopper powder produced by atomization is internally oxidized by heat treatment in oxidizing conditions and the powder is, thereafter reduced in conditions which turn the copper oxide back to copper without affecting the beryllium oxide. Such oxidized powders are then consolidated by pressing and sintering and working into the desired shapes. These methods of manufacture are expensive and time consuming and dispersion strengthened materials are therefore expensive to produce.

In addition to the above techniques, it has been proposed to melt a large bath of host metal or alloy which contains a small proportion of the reactive constituent required for the final dispersant. The bath is melted firstly under inert or reducing conditions in an induction furnace which promotes violent stirring. Atmospheric conditions are then adjusted so that the reactive metal oxidizes while the basic metal or alloy is unaffected and finally the melt, containing a fine dispersion of oxide, is cast into an ingot which can be worked by conventional methods. This technique is, however, difficult to operate.

It will be appreciated that the methods of making dispersion strengthened metals and alloys are complicated and time consuming and, moreover, the use of metals and alloys in powder form serves to render the cost of ingots made by such methods very high and uneconomic.

According to this invention, a method of making a dispersion strengthened metal or alloy comprises spraying a molten metallic host material and a reactive constituent on to a target, at least the reactive constituent passing through an atmosphere which serves to convert the reactive constituent into a material which forms a dispersed phase contained within the host material when the host material is deposited on the target.

In order to promote rapid solidification of the sprayed material, the target is preferably cooled. Alternatively, the target may possess a high thermal capacity. Spraying may be accomplished using flames, arcs, furnaces, plasmas and other spraying techniques.

The invention also includes a method of making a dispersion strengthened metal or alloy ingot suitable for subsequent fabrication, the method comprising the following steps: 7 l. producing a metallic starting material comprising a metallic host material and a relatively minor concentration of a metal or metals which is/are more reactive than the host material;

2. spraying finely divided molten particles of the starting material in the form of a jet through an atmosphere which reacts rapidly with the said more reactive constituent to form one or more stable metal compounds; 3. directing the sprayed jet of molten particles on to a cooled target or mould to build-up an ingot, and

4. removing the ingot from the target or mould and, thereafter, working the ingot into rod, wire, sheet or bar, or, into forged shapes.

Where it is required to form a dispersion strengthened alloy, the metallic host material need not necessarily be prepared before spraying. If required,

spraying can be accomplished using a mixture of metal powders (that is the constituents of the alloy) which will alloy when fused and before they are deposited on to the target. A

The invention also includes dispersion strengthened metals and alloys made by the method outlined above.

A dispersion strengthened metal or alloy made in accordance with this invention'has a relatively fine grain size which is determined to a large extent by the dimensions of the sprayed particles. Examination of alloys and metals made in'accordance with the invention indicated a fine dispersion of the dispersed phase and, since the reactive constituent from which the dispersed phase is formed, is molten at the same time as the host material, the reactive constituent solidifies to form the dispersed phase under conditions which approach thermodynamic equilibrium. Thus, if the host material has any tendency to reduce the oxide, as to some extent all metals do, this tendency should have been satisfied in the molten condition, so that no further reactions are likely at temperatures below the melting point. The dispersed phase contained in alloys and metals made in accordance with the invention may be in the form of an oxide, carbide, nitride, or sulphide and, for the reasons stated above, such dispersed phases possess high stabilities even in a metal matrix.

Additional grain stabilization is achieved by the dissolved absorbed or entrapped gas films which are associated with the molten spray when it impinges on the target or on previously deposited metallic material and 'which are, thereafter, permanently entrapped within the metallic matrix.

The temperature of spraying the host material and reactive constituent is adjusted so that it lies above the melting point of the host material and below the melting point of an oxide or other compound produced by the reaction of the more reactive constituent of the metal or alloy with the surrounding atmosphere.

EXAMPLE 1 Sheet from: Pure Powder metallurgy Sprayed platinum ingot with 0.08% Zr alloy ingot ZrO, ingot Density grm/cc 21.45 21.24 21.32

Grain size (1 hr. 1400C) 0.05 mm 0.0311 mm 0.0215

Hardness VPN 38 53.5 59.6

Tensile strength at C (annealed) 10.0 tsi 10.53 tsi 15.45 tsi Creep life at 1400C at tensile stress of 700 psi zfihour 40 hours 93 hours From the results set out in the table above, it will be appreciated that substantial improvements are obtained when the teaching of this invention is applied to platinum group metals.

EXAMPLE 2 Titanium platinum alloys were employed for some of the first systematic investigations, which were made to confirm the benefits obtainable. Platinum alloys containing 0.08 percent by weight of titanium were made up in a vacuum furnace and cast into ingots ranging in weight from 371 700 gms. These ingots were then cold rolled and finally drawn to wire 1 mm. in diameter which was sprayed into ingots, as before described in water cooled copper moulds. No sintering treatments were necessary, the ingot densition averaging rather more than 90 percent of theoretical before forging. Creep tests were carried out on 1 mm. diameter wire drawn from these ingots, the results obtained being summarized below:

Creep tests on 1 mm. dia. wire drawn from sprayed platinum ingots containing 0.08% by wt. of titanium Vacuum melted Rhodium platinum 280-320 20 Air Melted 10% rhodium Platinum 50 10 Although showing that substantial strengthening was being obtained the results were not completelyreproducible. Microstructural examination of some of the platinum strengthened with titanium oxide dispersions explained the erratic behavior. Round the grain boundaries of this material occasional layers of titanium dioxide could be detected and it appeared that under the spraying conditions provided with a conventional oxy-acetylene spray gun, the titanium oxide, once formed, soon fused into globules which solidified in the ingot as thin grain boundary films thus producing regions of potential weakness. Stringers of relatively large titanium oxide particles were also found in hard drawn platinum wire and it appeared evident that titanium oxide at the temperature of spraying possessed considerable mobility.

EXAMPLE 3 Tests were therefore carried out on platinum alloyed with small quantities of base metals having oxides which were more refractory than those of titanium.

In order to carry out these tests, ingots weighing 10 ounces were sprayed from wires containing small quantities of zirconium thorium and calcium. One ingot of 10 percent rhodium platinum containing nominally 0.4 volume per cent of zirconium oxide was also built up for purposes of comparison.

The platinum alloy wires used for building up these ingots were prepared from argon arc furnace ingots. Fabrication of the calcium platinum ingot was rather troublesome, although the zirconium and thorium bearing alloys presented no problem during melting and fabrication.

The stress rupture data on 1 mm. diameter wires prepared from these sprayed ingots is summarized below:

These results indicated the superiority of zirconium over titanium as a base metal alloying addition. To provide added confirmation six 50 ounce ingots were then sprayed from platinum alloy wires alloyed with sufficient zirconium to yield material, which, after deposition contained 0.4 percent by volume of refractory oxide.

These alloy ingots, produced in the vacuum furnace were rolled and drawn down to wires 0.058 inch in diameter which were sprayed with an oxy-acetylene spray gun into water cooled copper moulds to produce rectangular ingots 8 inches long, 0.75 inch wide and 0.5 0.75 inch thick.

Life at 1400 psi Life at 2800 psi (Hrs.) (Hrs.)

lngot No. of Average No. of Average No. Tests Life Range Tests Life Range P4870 3 131 110-154 3 11 4-21 P4875 3 310 186-424 3 11 9-12 P503D 3 190 130-251 2 14 12-16 P503C 3 485 425-551 2 29 21-36 P5031 2 597 454-740 2 64 56-71 P487G 2 696 488-903 2 36 29-42 These test results provided overwhelming confirmation that zirconium additions were superior to titanium additions, even minimum test lives observed being considerably greater than those exhibited by very strong conventional rhodium platinum alloys such as those containing 25 percent by wt. of rhodium.

EXAMPLE 5 The next step was to assess the quality of sheet produced from these 50 ounce ingots. A small portion only of the ingots had been used for the wire, the test results of which are summarized in the above Table. The remainders of some of these ingots were therefore hot rolled and finally cold rolled to produce sheet 0.060 inch thick from which test-pieces were prepared. A summary of the high temperature behavior of the sheet so obtained is provided below:

Stress rupture data on dispersion strengthened platinum sheet 0.060 inch thick tested in tension in air at 1400 C. Sheet rolled from 50 ounce sprayed ingots containing 0.1 percent by weight of zirconium.

lngot No. Life in Hours 700 p.s.i. 1400 p.s.i. P487E Test Piece 362 16-30 Cross Section P503D 0.25" x 0.060" 455 27-42 P5031 400 70-90 P4876 473 73-76 Conventionally 150-300 -15 produced 25% rhodium platinum sheet The minimum test life obtained on this dispersion strengthened platinum sheet was therefore superior to that of the standard 25 percent rhodium plated alloy, although sheet test results were rather lower than those obtained on wires. This is a well known characteristic of dispersion strengthened materials, the creep properties of which vary considerably with the extent and type of plastic deformation to which they are submitted during fabrication.

EXAMPLE 6 Two gold alloys were made up by induction melting in a graphite crucible. Melting was carried out under an argon atmosphere and to one batch of gold 0.80 percent of titanium was added, 0.04 percent of aluminum Resistance to Creep Failure of Flame Sprayed Gold Alloys (Tests made at 700 p.s.i., at 700C in Air) 0.04% Al Pure Gold 7 Base Metal Addition Life in Hours The two flame sprayed alloys were therefore at 700 C very much stronger than pure gold. Bend tests were also carried out on these wires to assess their formability at room temperature. Wires 0.0180 inch diameter were lightly clamped between two polished steel jaws with corners rounded to a radius of l/32nd of an inch. The wireswere then bent through 90 in each direction and back through 180 in the same plane at right angles to the interface between the metal jaws. Each 180 bend constituted a complete reversal of the strain applied to the wire. Bending was carried out until the wires broke. The following test results were obtained:

Reverse bends before failure Wire Work Hardened Annealed Sprayed gold with 0.08% Ti 8-16 10-16 Sprayed gold with 0.04% Al 9-16 10-16 Pure gold from conventional 5-1 1 6-13 ingot cast The gold wire from the sprayed ingots was therefore considerably more resistant to failure in bend testing than pure gold and this superiority was evident even when work hardened wires were tested.

WORKING CHARACTERISTICS OF DISPERSION STRENGTHENED PLATINUM MADE FROM FLAME SPRAYED INGOTS The workability of flame sprayed dispersion strengthened platinum was soon found to be superior to that of the rhodium platinum alloys it was intended to displace. This characteristic is illustrated in FIG. 1 of the accompanying drawings where the work hardening characteristics of flame sprayed platinum are comof the metals and alloys is phitted against the true strain e which is a measure of the total change in If L is the original length and L the final length,

e=Log (1+ SOFTENING AND ANNEALING CHARACTERISTICS Some of the effects of isochronal annealing on flame sprayed dispersion strengthened platinum are illus trated in FIG. 2 of the accompanying drawings. These annealing tests were made on different batches of material to those used for the work hardening experiments so the initial hardness values do not correspond exactly to those which would be derived from FIG. 1. The four samples of sheet, pure platinum, powder metallurgically produced dispersion strengthened platinum, flame sprayed dispersion strengthened platinum and conventional percent rhodium platinum sheet made from a melted and cast ingot were all fully annealed for one hour at 1 ,200 C. They were then subjected, by cold rolling to a true strain of 2.0 and annealed for 30 minute intervals at temperatures ranging from 200 to l ,400 C.

The pure platinum began to soften at about 300 C, whereas significant changes in hardness did not occur with either of the two dispersion strengthened grades of platinum below 400 C. The softening which occurred at high temperatures was gradual, constant hardness values not being achieved below l,200 C.

The 10 percent rhodium platinum alloy softened completely in the narrow temperature range between 700 C and 800 C.

ELECTRICAL PROPERTIES OF FLAME SPRAYED DISPERSION STRENGTHENED PLATINUM The electrical resistivity of dispersion strengthened platinum is compared below with that of pure platinum and the standard type of 10 percent rhodium platinum alloy.

Material Electrical Resistivity at 20C (p. (1 cm) Flame sprayed Pt (0.1% Zr) [1.6

Pure platinum from strengthened platinum is therefore less than ten per cent higher than that of pure platinum. This ability to improve the mechanical properties of any desired metal or alloy without any serious change in their electrical characteristics is a great advantage of dispersion strengthening, and it has been shown that several of the noble metal electrical resistance alloys can be produced with greatly improved mechanical properties by flame spraying techniques.

The following indicate some uses or applications of flame sprayed dispersion strengthened metals:

1. Structural units made in accordance with the invention which are required to withstand high stresses for long periods at temperatures close to their melting points.

2. Articles as above made from platinum or platinum alloys.

3. Articles suitable for use in contact with molten glass at high temperatures.

4. Thermocouple and other temperature measuring devices.

5. Electrical contacts.

6. Dispersion strengthened silver springs and/or contacts where the material must have a low electrical resistivity to allow it to carry heavy electrical currents, where it must have a low elastic modulus and high elastic limit to allow it to perform effectively as a spring, and where the use of a noble metal is essential to maintain contact resistances at a low level.

7. Gold alloy springs/contacts/current conductors as above.

8. Resistance thermometers where high mechanical strength at high temperatures must be accompanied by a high temperature coefficient of resistance which is usually restricted to a pure metal.

9. High temperature catalyst gauzes for ammonia oxidation and other purposes.

l0. Dispersion strengthened palladium-gold and other alloy catchment gauzes used in conjunction with nitric acid gauzes.

ll. Dispersion strengthened noble metal slidewire resistances. These have the advantage that mechanical properties could be substantially improved with little sacrifice of electrical characteristics.

l2. Dispersion strengthened alloy heater elements based on the nickel chromium system.

13. Dispersion strengthened alloy heater elements based on the iron-chromium-aluminum system.

14. Dispersion strengthened noble metal heater elements used for igniting coal and natural gas.

15. Dispersion strengthened noble metal heater elements used in electrical furnaces.

l6. Dispersion strengthened palladium diffusion membranes.

l7. Dispersion strengthened silver-palladium and other palladium alloy diffusion membranes for the separation and purification of hydrogen.

l8. Dispersion strengthened palladium and palladium alloy diffusion membranes used for the elec' trolytic separation and concentration of deuterium.

19. Dispersion strengthened palladium and palladium alloy diffusion membranes used for the gas phase separation and concentration of deuterium.

20. Base and noble metal igniter wires used in electrical detonators and other devices for the initiation of explosions.

21. Dispersion strengthened base metal and noble metal and alloy spinnerettes used for the manufacture of synthetic fibers. Our investigations have, however, shown that the teaching of the invention is not confined to the noble metals but that it is also applicable to other metals and alloys. For example, beryllium/copper and other high duty copper alloys, may be sprayed into ingots as outlined in this specification. Moreover, high alloy steels may be sprayed under carbonizing conditions so that a sprayed deposit with a fine dispersion of tungsten, titanium, zirconium or chromium carbide is obtained.

The invention also includes articles when made in accordance with the invention and method described above. The articles may be in sheet, rod or wire form and such articles have high corrosion resistance.

What is claimed is: l. A method of making a dispersion strengthened metal or alloy comprising spraying (i) a molten metallic host material selected from the group consisting of metals and alloys, and (ii) a molten reactive constituent, on to a target; passing said molten reactive constituent, prior to its reaching said target, through an atmosphere which converts said molten reactive constituent into a converted constituent which forms on cooling a dispersed phase within the host material;

said host material and said converted constituent being substantially simultaneously deposited on the target to form an ingot of said host material having converted constituents dispersed therethrough; and

removing said ingot and working and fabricating said ingot.

2. A method according to claim 1 wherein the target is cooled.

3. A method according to claim 1 wherein the target is formed of a material having a high thermal capacity.

4. A method according to claim 1 wherein the target is in the form of a mold inside which an ingot of definite shape can be built up.

5. A method according to claim 1 wherein said target is cooled, wherein said molten reactive constituent is a metal more reactive than said host material, and wherein said host material and said reactive constituent are admixed; and spraying the admixture of host material and reactive constituent in the form of a jet of finely divided molten particles through an atmosphere which reacts with said molten reactive constituent to form at least one stable metal compound as the said converted constituent during passage of said reactive constituent through said atmosphere.

6. A method according to claim 5 wherein the host material comprises an alloy, which includes the step of spraying a mixture of metal powders, forming the constituents of the alloy, adapted to alloy when in the molten state prior to deposition.

7. A method according to claim 6 wherein the atmosphere is such as to provide a dispersed phase of the converted reactive constituent selected from at least one member of the group consisting of an oxide, carbide, nitride and sulphide.

8. A method according to claim 5 wherein said admixture is an alloy of said host material and said reactive constituent.

9. A method according to claim 8 wherein said atmosphere is such as to react with said reactive constituent to form a converted constituent selected from the group consisting of an oxide, carbide, nitride and sulphide of said reactive constituent.

10. A method according to claim 9 wherein the temperature of spraying is adjusted so that it lies above the melting point of the host metal, and below the melting point of said converted reactive constituent.

11. A method according to claim 5 wherein said metallic host material is selected from the group consisting of platinum group metals and alloys containing at least one metal of said group.

12. A method according to claim 11 wherein said host material is platinum or an alloy of platinum with a minute portion of another platinum group metal, wherein said reactive constituent is one which reacts with oxygen, carbon, nitrogen or sulphur to form an oxide, carbide, nitride, or sulphide, and wherein the temperature of spraying is above the melting point of said host material and below the melting point of said converted constituent.

13. A method according to claim 12 wherein said host material is platinum or a platinum-rhodium alloy and wherein said reactive constituent is selected from the group consisting of titanium, zirconium, thorium, and calcium.

14. A method according to claim 13 wherein said reactive constituent is zirconium.

15. A method according to claim 8 wherein said host material is gold.

16. A method according to claim 15 wherein said reactive constituent is selected from the group consisting of titanium and aluminum. 

2. A method according to claim 1 wherein the target is cooled.
 3. A method according to claim 1 wherein the target is formed of a material having a high thermal capacity.
 4. A method according to claim 1 wherein the target is in the form of a mold inside which an ingot of definite shape can be built up.
 5. A method according to claim 1 wherein said target is cooled, wherein said molten reactive constituent is a metal more reactive than said host material, and wherein said host material and said reactive constituent are admixed; and spraying the admixture of host material and reactive constituent in the form of a jet of finely divided molten particles through an atmosphere which reacts with said molten reactive constituent to form at least one stable metal compound as the said converted constituent during passage of said reactive constituent through said atmosphere.
 6. A method according to claim 5 wherein the host material comprises an alloy, which includes the step of spraying a mixture of metal powders, forming the constituents of the alloy, adapted to alloy when in the molten state prior to deposition.
 7. A method according to claim 6 wherein the atmosphere is such as to provide a dispersed phase of the converted reactive constituent selected from at least one member of the group consisting of an oxide, carbide, nitride and sulphide.
 8. A method according to claim 5 wherein said admixture is an alloy of said host material and said reactive constituent.
 9. A method according to claim 8 wherein said atmosphere is such as to react with said reactive constituent to form a converted constituent selected from the group consisting of an oxide, carbide, nitride and sulphide of said reactive constituent.
 10. A method according to claim 9 wherein the temperature of spraying is adjusted so that it lies above the melting point of the host metal, and below the melting point of said converted reactive constituent.
 11. A method according to claim 5 wherein said metallic host material is selected from the group consisting of platinum group metals and alloys containing at least one metal of said group.
 12. A method according to claim 11 wherein said host material is platinum or an alloy of platinum with a minute portion of another platinum group metal, wherein said reactive constituent is one which reacts with oxygen, carbon, nitrogen or sulphur to form an oxide, carbide, nitride, or sulphide, and wherein the temperature of spraying is above the melting point of said host material and below the melting point of said converted constituent.
 13. A method according to claim 12 wherein said host material is platinum or a platinum-rhodium alloy and wherein said reactive constituent is selected from the group consisting of titanium, zirconium, thorium, and calcium.
 14. A method according to claim 13 wherein said reactive constituent is zirconium.
 15. A method according to claim 8 wherein said host material is gold.
 16. A method according to claim 15 wherein said reactive constituent is selected from the group consisting of titanium and aluminum. 